With moby/buildkit#2879 close to merging, I thought I'd propose some new syntax here 🎉

When merged, it'll be possible to use annotations as follows (with no changes to buildx):

docker buildx build . --output type=image,name=example/foo,push=true,annotation.x=1 --platform=linux/arm64,linux/amd64

However, this isn't ideal, and it would be nice to provide a shorthand, similar to how --push appends push=true.

Buildx

For buildx, we can provide the --annotation flag:

docker buildx build . --output type=image,name=example/foo,push=true --annotation x=1 --platform=linux/arm64,linux/amd64

For platform-specific annotations, we can allow prefixing with the platform name:

docker buildx build . --output type=image,name=example/foo,push=true --annotation linux/amd64:x=1 --platform=linux/arm64,linux/amd64

Not super sure about the exact syntax, but something along those lines should work.

I think we should explicitly avoid allowing using anything other than the manifest annotation type, directing users to manually add the output attribute if they want more advanced behavior.

Bake

For bake, we can add an annotations key:

target "x" {
    ...
    annotations = {
        "linux/amd64:x" = "1"
    }
    ...
}

Comments appreciated 🎉